This invention relates to a Direct Electrostatic Printer (DEP) device and more particularly to a toner delivery system for presenting charged toner to an electronically addressable printhead utilized for depositing toner in image configuration on plain paper.
Of the various electrostatic printing techniques, the most familiar is that of xerography wherein latent electrostatic images formed on a charge retentive surface are developed by a suitable toner material to render the images visible, the images being subsequently transferred to plain paper.
A less familiar form of electrostatic printing is one that has come to be known as direct electrostatic printing (DEP). This form of printing differs from the aforementioned xerographic form, in that, the toner or developing material is deposited directly onto a plain (i.e. not specially treated) substrate in image configuration. This type of printing device is disclosed in U.S. Pat. No. 3,689,935 issued Sept. 5, 1972 to Gerald L. Pressman et al.
Pressman et al disclose an electrostatic line printer incorporating a multilayered particle modulator or printhead comprising a layer of insulating material, a continuous layer of conducting material on one side of the insulating layer and a segmented layer of conducting material on the other side of the insulating layer. At least one row of apertures is formed through the multilayered particle modulator. Each segment of the segmented layer of the conductive material is formed around a portion of an aperture and is insulatively isolated from every other segment of the segmented conductive layer. Selected potentials are applied to each of the segments of the segmented conductive layer while a fixed potential is applied to the continuous conductive layer. An overall applied field projects charged particles through the row of apertures of the particle modulator and the density of the particle stream is modulated according to the pattern of potentials applied to the segments of the segmented conductive layer. The modulated stream of charged particles impinge upon a print-receiving medium interposed in the modulated particle stream and translated relative to the particle modulator to provide line-by-line scan printing. In the Pressman et al device the supply of the toner to the control member is not uniformly effected and irregularities are liable to occur in the image on the image receiving member. High-speed recording is difficult and moreover, the openings in the printhead are liable to be clogged by the toner.
U.S. Pat. No. 4,491,855 issued on Jan. 1, 1985 in the name of Fujii et al discloses a method and apparatus utilizing a controller having a plurality of openings or slit-like openings to control the passage of one-component insulative magnetic toner and to record a visible image by the charged particles directly on an image receiving member. Fuji, et al. show an apertured printhead structure having wedge-shaped apertures wherein the larger diameter of an aperture is delineated by a signal or control electrode and is disposed opposite an image receiving substrate.
U.S. Pat. No. 4,568,955 issued on Feb. 4, 1986 to Hosoya et al discloses a recording apparatus wherein a visible image based on image information is formed on an ordinary sheet by a developer. The recording apparatus comprises a developing roller spaced at a predetermined distance from and facing the ordinary sheet and carrying the developer thereon. It further comprises a recording electrode and a signal source connected thereto for propelling the developer on the developing roller to the ordinary sheet by generating an electric field between the ordinary sheet and the developing roller according to the image information. A plurality of mutually insulated electrodes are provided on the developing roller and extend therefrom in one direction. An A.C. and a D.C. source are connected to the electrodes, for generating an alternating electric field between adjacent ones of the electrodes to cause oscillations of the developer found between the adjacent electrodes along electric lines of force therebetween to thereby liberate the developer from the developing roller. In a modified form of the Hosoya et al device, a toner reservoir is disposed beneath a recording electrode which has a top provided with an opening facing the recording electrode and an inclined bottom for holding a quantity of toner. In the toner reservoir are disposed a toner carrying plate as the developer carrying member, secured in a position such that it faces the end of the recording electrode at a predetermined distance therefrom and a toner agitator for agitating the toner.
U.S. Pat. No. 4,814,796 granted to Fred W. Schmidlin on Mar. 21, 1989 describes a DEP apparatus including a structure for delivering developer or toner to an apertured printhead structure. The toner delivery system of this patent delivers toner containing a minimum quantity of wrong sign and size toner. To this end, the developer delivery system includes a conventional magnetic brush which delivers toner to a donor roll member which, in turn, delivers toner to the vicinity of the apertures in the printhead structure.
U.S. Pat. No. 4,743,926 granted to Schmidlin et al on May 10, 1988 describes a developer or toner delivery system adapted to deliver toner containing a minimum quantity of wrong sign and size toner. To this end, the delivery system includes a pair of charged toner conveyors which are supported in face-to-face relation. An electrical bias is applied across the two conveyors to cause toner of one polarity and size to be attracted to one of the conveyors while toner of the opposite polarity is attracted to the other of the two conveyors. Only toner from one of the conveyors is delivered to an apertured printhead forming a part of a DEP apparatus.
U.S. Pat. No. 4,876,561 granted to Fred W. Schmidlin on Oct. 24, 1989 describes a DEP device which is optimized by presenting well charged toner to a charged toner conveyor which conveys the toner to an apertured printhead. The charged toner conveyor comprises a plurality of electrodes wherein the electrode density (i.e. over 400 electrodes per inch) is relatively large for enabling a high toner delivery rate without air breakdown. To this end, the thickness of the printhead structure is about 0.025 mm and the aperture diameter (i.e. 0.15 mm) is large compared to the printhead thickness.
U.S. Pat. No. 4,903,049 decribes Direct Electrostatic Printing which is enhanced by the provision of wrong sign toner extraction holes in a printhead structure at a location which is upsteam of the printing apertures. Wrong sign toner particles are extracted from a cloud of toner provided from a toner delivery device. The wrong sign toner is extracted from the powder cloud before the cloud reaches the vicinity of the printing apertures thereby minimizing print hole blockage.
U.S. Pat. No. 4,903,050 describes Direct Electrostatic Printing which is enhanced by the provision of a shutter mechanism for preventing toner dislodged from the backside of a printhead structure from being deposited on image receiving substrates. The shutter is interposed between the printhead structure and the image receiving substrate during a cleaning cycle. The dislodged toner is removed from the between the printhead structure and the image receiving substrate with a combination vacuum and toner collection device.
The use of an airborne toner source that is capable of delivering toner into proximity with the apertures in an apertured printhead structure of a DEP apparatus is essential to enable extended printing without the ruse of periodic cleaning of the printhead structure. Also, it is desirable that the toner source be simple, low cost, reliable and have an extended operating life. Further, toner charged to a specific level of charge/mass is especially desired for DEP because it controls the passage of toner through the holes in the apertured printhead.